1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus which includes a gradient system for generating a magnetic gradient field in an imaging volume of the apparatus, which gradient system includes a primary gradient coil for generating the gradient field and a shielding coil for actively shielding the space outside the imaging volume from the magnetic field generated in said space by the primary gradient coil.
2. Description of the Related Art
An apparatus of this kind is known from U.S. Pat. No. 5,349,297
A magnetic resonance imaging apparatus for medical purposes, also referred to as MRI apparatus, is arranged to form images of cross-sections of a body. To this end, a strong, steady, uniform field is generated in a volume in such an apparatus which is intended for imaging, i.e. the so-called imaging volume. A gradient field is superposed on said uniform field in order to indicate the location of the cross-section to be imaged. The atoms in the tissue present in the imaging volume are then excited by means of an RF field; the spin resonance signal produced upon relaxation of the excited atoms is then used to reconstruct an image of the cross-section indicated by the gradient field. The steady, uniform field, also referred to as the main field, is generated by means of a coil system (superconducting or not). Together with the associated envelope this coil system is shaped as a short tube in which the imaging volume is situated. The diameter of this tube is determined by the dimensions of the patients to be examined and hence has a given minimum value, for example of the order of magnitude of 90 cm.
The gradient system for generating the magnetic gradient field in the imaging volume is arranged within said tube and around the imaging volume. The gradient system includes gradient coils for generating an associated gradient field; for each of the three co-ordinate directions it includes one set, each of which is referred to as a primary gradient coil. Thus three axial fields are produced with gradients in the three co-ordinate directions x, y and the axial direction z. Current pulses are applied to the gradient coils during the imaging process, so that inevitably magnetic stray fields are produced also outside the imaging volume.
These stray fields are capable of inducing eddy currents in the conductive parts of the apparatus which are present in the vicinity of the gradient coil, notably the metal parts provided for generating the main field, for example the tubular part of the envelope for the coil system, any thermal shields situated within the envelope (in the case of a superconducting coil system) or the coils for the main field themselves. The magnetic fields generated by the eddy currents cause distortions of the image to be formed. Moreover, they cause heat dissipation in the parts carrying the eddy current; this is a drawback notably in the case of a superconducting coil system, because the liquid helium acting as the cooling medium then boils off faster. Finally, they also cause an annoying noise in that the parts conducting the eddy current are situated in a magnetic field and hence are subject to Lorenz forces which cause deformation of said parts.
In order to counteract the above-mentioned adverse effects of the eddy currents, attempts are made to shield or compensate the magnetic fields generated outside the imaging volume by the primary coil. From the cited United States patent it is known that in order to compensate said stray fields a shielding coil is arranged around the primary gradient coil, said shielding coil being intended to provide active shielding of the space outside the imaging volume from the magnetic field generated in said space by the primary coil by compensation. Active shielding is to be understood to mean herein the shielding by generating a compensating magnetic field by means of a shielding coil other than the primary coil. Such a shielding coil can be connected in series with the primary coil or be controlled independently of the primary coil; the latter case is referred to as independent active shielding.
Even though a shielding coil for active shielding offers a substantial reduction of the stray field, in practical circumstances a given amount of stray field always remains. This can be explained in that inter alia the shielding coil is made of discrete turns and in that dimensional deviations inevitably occur due to manufacturing tolerances, so that magnetic flux can still escape, between the shielding turns, to the area surrounding the system formed by the gradient coil and the shielding coil.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.